1. Field of the Invention
The present invention relates to an improvement in an ink jet head for discharging ink which is applied to an ink jet system. More particularly, the present invention relates to an improved ink jet head free of debonding between a substrate for an ink jet head and ink flow path walls formed on said substrate. The present invention includes an ink jet apparatus which is provided with said ink jet head.
2. Description of Related Background Art
In recent years, progressively increasing attention has been paid to an ink jet system for conducting printing on a printing medium by discharging ink droplets towards said printing medium, since it can provide high speed, high density and high definition printing of a high quality with low noise and is suitable for color printing and also for compact designing.
An ink jet head which serves as a mechanism for discharging ink in such ink jet system is the most important factor in the ink jet system. The ink jet head has an extremely precise structure. Description will be made of such ink jet head with reference to FIG. 4. FIG. 4 is a schematically exploded view illustrating an example of the construction of the ink jet head.
In FIG. 4, reference numeral 101 indicates a silicon substrate for an ink jet head, having an electrothermal converting element as a discharging energy-generating element including head generating resistors 103 and wiring 102 for said heat generating resistors formed on said silicon substrate by means of the thin-film forming technique. On the silicon substrate 101 having the electrothermal converting element thereon, there is disposed an ink flow path wall made of a photosensitive resin or the like, forming ink pathway walls 104a and a common ink chamber wall 104b. Reference numeral 105 indicates a top plate joined onto the ink flow path wall 104 such that it covers concavities formed by the ink flow path wall 104 to form ink pathways 108a and a common ink chamber 108b. Each ink pathway 108a contains the above-described heat generating resistor 103 positioned therein. Reference numeral 109 indicates an ink discharging outlet positioned at an end portion of the silicon substrate which communicates with the ink pathway 108a. Reference numeral 107 indicates a common ink supply port disposed at the top plate 105. The common ink supply port 107 is covered by a filter 106 for ink which is fixed to the top plate 107.
There are various proposals for the process of producing an ink jet head of the construction above described. U.S. Pat. No. 4,657,631 describes a typical process for producing such an ink jet head.
The process described in this patent document will be described with reference to FIG. 1.
The process for producing an ink jet head according to the above patent document comprises the following six steps:
step 1 (see, FIG. 1(a)) wherein a substrate 1 for an ink jet head which is provided with an energy generating element serving for discharging ink is provided, and a photosensitive resin layer 2 (comprising a positive type photoresist) is formed on the substrate 1;
step 2 (see, FIG. 1(b)) wherein the photosensitive resin layer 2 formed in step 1 is subjected to light exposure through a patterning mask;
step 3 (see, FIG. 1(c)) wherein the photosensitive resin layer 2 having been subjected to light exposure in step 2 is patterned by way of subjecting said resin layer, whereby a plurality of relief solid layers 4 (that is, a plurality of relief patterns) are formed at respective positions on the substrate 1 where ink flow paths are to to be formed;
step 4 (see, FIG. 1(d)) wherein a ink flow path wall-forming material 5 comprising a hardening resin is disposed to cover the relief solid layers 4 formed in step 3;
step 5 (see, FIG. 1(e)) wherein the hardening resin is hardened; and
step 6 (see, FIG. 1(f)) wherein the relief solid layers 4 are removed by way of dissolving them using an appropriate organic solvent such as halogen-containing hydrocarbon, ketone, ester, or alcohol or an aqueous alkali solution of potassium hydroxide or the like; whereby an ink jet head is completed.
In the above-described process for producing an ink jet head, there is usually used a hardening resin as the ink flow path wall-forming material 5 as above described. However, a problem arises in this case in that a cure shrinkage is liable to occur upon hardening of the hardening resin, wherein shrinkage stress is unavoidable. Also, such hardening resin is somewhat swollen when contacted with ink. This swelling of the hardening resin provides a stress in accordance with a change in the environmental temperature, particularly upon operating the ink jet head.
In the case of an ink jet head having ink flow path walls comprised of the hardening resin, debonding is liable to occur between the ink flow path walls and the substrate when the ink jet head is repeatedly used, because of the above described stress. This debonding entails a loss in the discharging energy to the ink pathways, wherein in the worst case, the discharging energy becomes insufficient such that no ink is discharged from a discharging outlet. Therefore, in the production of an ink jet head, an appropriate ink flow path wall-forming material is selectively used in order to prevent in the ink flow path walls from being debonded from the substrate.
In order that the ink flow path walls are prevented from being debonded from the substrate, there is known a method wherein wherein the ink flow path walls are formed with the use of an ink path wall-forming material having a reduced elasticity modulus so that they can absorb the foregoing stress provided in accordance with the change in the environmental temperature and another method wherein the ink flow path walls are formed with the use of an ink path wall-forming material which is slightly swollen with ink so that they slightly provide such stress due to a change in the environmental temperature. However, neither of these two methods is sufficiently effective in practice. That is, such ink flow path wall-forming material having a reduced elasticity modulus is relatively large in terms of the swelling magnitude to ink and because of this, the ink flow path walls in accordance with either of the two methods are insufficient in terms of their stress absorbing efficiency.
Thus, it is necessary for the ink flow path walls to be formed of a specific hardening resin having a relevant elasticity modulus while satisfying conditions relating to the swelling property to ink and the stress absorbing efficiency.
Now, the swelling property of the ink flow path wall-forming material to ink can be referred to as "ink resistance". An ink flow path wall-forming material having a high swelling property to ink, can be considered to be low in ink resistance. On the other hand, an ink flow path wall-forming material having a low swelling property to ink can be considered to be high in ink resistance.
As described above (see, FIG. 1(d)), the ink flow path wall-forming material is applied to cover the relief solid layers (that is, the relief patterns), for example, by means of a dispenser. In the case where a plurality of ink pathways are intended to be arranged at an increased density, it is necessary for the relief patterns to be miniaturized in order to attain this purpose. In this case, the ink flow path wall-forming material is required to be of a low viscosity so that it can sufficiently get into spaces among those miniaturized relief patterns. The use of an ink flow path wall-forming material having a low viscosity in this case is advantageous in that air bubbles are rarely taken in upon the application of thereof. However, the relief patterns (that is, the relief solid layers), which serve to form a plurality of ink pathways, are usually formed of a positive type photoresist as described above. Thus, in the case where the ink flow path wall-forming material contains an organic solvent, the shapes of the relief solid layers are liable to be deformed because of the organic solvent, wherein precise ink pathways cannot be formed. Therefore, in order for the ink flow path wall-forming material to have a desired viscosity, it is desired to use an appropriate solvent-free material in the liquid state at room temperature and which has a low viscosity and is capable of serving as the ink flow path wall-forming material.
In the prior art, epoxy resin has been used as the ink flow path wall-forming material since it has excellent ink resistance and in the liquid state at room temperature. The epoxy resin is corresponding to the foregoing ink flow path wall-forming material has excellent ink resistance but has a high elasticity modulus. In the case of an ink jet head having a plurality of ink flow path walls formed of epoxy resin, there is a problem in that the ink flow path walls are sometimes debonded from the substrate. Particularly, in the case of producing an ink jet head in which a plurality of energy generating elements are arranged at an increased density, the interval between each adjacent energy generating element is remarkably shortened and because of this, the width of each ink flow path wall comprised of epoxy resin is necessary to be thinned accordingly. In this case, there is a problem in that because the area of each ink flow path wall to be contacted with the substrate eventually becomes small, a sufficient adhesion is difficult to maintain between the ink flow path wall and the substrate. In addition to this, another problem is in that because the epoxy resin has a high elasticity modulus, a shrinkage stress effected upon hardening it cannot be sufficiently absorbed, often resulting debonding between the ink flow path walls and the substrate.
In order that the epoxy resin can absorb the foregoing shrinkage stress, the epoxy resin must be modified such that it has a reduced elasticity modulus. For this purpose, there can be considered a method wherein the epoxy resin is replaced by a silicone-modified epoxy resin. However, the silicone-modified epoxy resin is remarkably low in elasticity modulus and because of this, there cannot be attained the desired bonding strength required for the ink flow path wall-forming material. In addition to this, the silicone-modified epoxy resin is insufficient in terms of mechanical strength. In this connection, silicone-modified epoxy resin has never been used as a constituent material of an ink jet head. In order for the epoxy resin to have a reduced elasticity modulus, an appropriate flexibilizer is usually incorporated thereinto.
In the ink jet system field in recent years, there have been used as the printing medium various printing members such as plastic sheets, fabrics and the like other than paper. Along with this, there is an increased demand for improving ink to be used in the ink jet system in terms of its color development and water resistance. In order to meet this demand, there is often used ink containing a slightly water-soluble dye or/and a pigment which is low in dispersion stability instead of ordinary ink.
However, full use of such dye and pigment cannot be attained in the case where an ordinary solvent is used. In order to solve this problem, there is a proposal of incorporating an alkali salt such as lithium hydroxide into the solvent to heighten the pH value of the ink (that is, to make the ink highly alkaline) whereby making full use of such dye and pigment.
The results of experimental studies by the present inventors revealed that a conventional ink jet head having a plurality of ink flow path walls formed of an epoxy resin containing a flexibilizer is liable to provide prints accompanied by defects when it is operated using highly alkaline ink over a long period of time and wherein debonding is liable to occur between the ink flow path walls and the substrate. This situation will be detailed later.